Photopolymerizable Flexographic Printing Elements for Printing with UV Inks

ABSTRACT

Photopolymerizable flexographic printing elements which contain cyclohexanepolycarboxylic esters as plasticizers and also their use for producing flexographic printing forms for printing with UV inks, in particular for UV narrow web printing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/993,182,filed Jul. 1, 2011, the entire contents of which is incorporated byreference herein. application Ser. No. 12/993,182 is a national stageapplication (under 35 U.S.C. §371) of PCT/EP2009/055775, filed May 13,2009, the entire contents of which is incorporated by reference herein.PCT/EP2009/055775 claims benefit of German application DE102008024214.4,filed May 19, 2008, the entire contents of which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

The present invention relates to photopolymerizable flexographicprinting elements which contain cyclohexanepolycarboxylic esters asplasticizers and also their use for producing flexographic printingforms for printing with UV inks, in particular for UV narrow webprinting.

Flexographic printing by means of UV inks is known in principle. Thetechnique is used, in particular, for producing labels. In UV narrow webprinting, a web, for example of paper or self-adhesive plastic films, isprinted with the label pattern. The printing machines used for thispurpose have printing cylinders having a relatively small diameter, forexample a diameter of only from 6 to 12 cm. When mounting on theprinting cylinders, the flexographic printing plates used accordinglyhave to be very flexible so that they can readily be bent around thesmall printing cylinder.

UV flexographic printing inks contain, for curing, monomers havingethylenically unsaturated groups, for example hexanediol diacrylate,dipropylene glycol diacrylate, tripropylene glycol diacrylate or elseethoxylated monomers such as ethoxylated tripropylene glycol diacrylateor ethoxylated trimethylolpropane triacrylate. Such monomers can swellflexographic printing plates over the course of time. However, swellingof the flexographic printing plate leads with increasing duration of theprinting operation to undesirable changes in the printed image becausefine print elements experience a volume increase due to the swelling andat the same time the hardness of the flexographic printing plate candecrease. The individual elements of the plate become broader, the shadevalue increase goes up and fine picture elements can run together.

The swellability of the flexographic printing plate is naturallyinfluenced by the degree of crosslinking. The higher the degree ofcrosslinking, the lower the swellability of the plate. Relatively highcrosslinking also increases the hardness of the flexographic printingplates under otherwise the same conditions, which can be thoroughlydesirable in halftone printing. At the same time, however, theflexibility of the flexographic printing plate also decreases, i.e. itis more difficult to bend so that mounting on the printing cylinderhaving a small diameter becomes difficult.

It has been proposed that specific binders be used in the flexographicprinting plates for printing with UV inks. EP 833 206 A and EP 833 207 Apropose using specific block copolymers having styrene blocks and alsoisoprene or butadiene-isoprene blocks, with the peak temperature of theprimary dispersion of tan δ being not more than 30° C. EP 992 849 A1proposes using block copolymers having at least one styrene block and atleast one styrene-butadiene block.

The addition of plasticizers to flexographic printing plates is known.On this subject, reference may be made by way of example to EP 992 849A1, paragraph [0021]. White oils or oligomeric plasticizers, inparticular polybutadiene oils, are particularly widespread asplasticizers in flexographic printing plates.

The flexibility of flexographic printing plates can in principle beincreased by the use of plasticizers. However, relatively large amountsof plasticizers at the same time also decrease the degree ofcrosslinking, so that the flexographic printing plate becomes softeragain and accordingly also swell to a greater degree again on contactwith UV inks.

It was an object of the invention to improve the flexibility offlexographic printing plates for printing with UV inks, in particular UVnarrow web printing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic depiction of the measurement of the flexibility.

DETAILED DESCRIPTION OF THE INVENTION

It has surprisingly been found that this object can be achieved by theuse of cyclohexanepolycarboxylic esters as plasticizers in flexographicprinting plates.

We have accordingly found a photomerizable flexographic printing elementfor producing flexographic printing forms for printing with UV inks,which comprises at least

-   -   a dimensionally stable support and    -   a photopolymerizable, relief-forming layer comprising at least        -   from 40 to 90% by weight of a thermoplastic-elastomeric            block copolymer comprising at least one block which consists            essentially of alkenylaromatics and at least one block which            consists essentially of 1,3-dienes,        -   from 1 to 20% by weight of ethylenically unsaturated            monomers,        -   from 0.1 to 5% by weight of photoinitiator and        -   from 1 to 40% by weight of plasticizer,    -   where the amounts are in each case based on the total amount of        all components of the photopolymerizable layer,    -   wherein at least one of the plasticizers is from 1 to 40% by        weight of at least one cyclohexanepolycarboxylic ester of the        general formula R¹—(COOR²)_(n) where n is 2, 3 or 4, R¹ is an        n-valent cyclohexane radical and the radicals R² are each,        independently of one another, a linear, branched or cyclic,        aliphatic hydrocarbon radical having from 3 to 20 carbon atoms.

In a preferred embodiment of the invention, thethermoplastic-elastomeric block copolymer is a styrene-isoprene blockcopolymer.

Furthermore, we have found a process for producing flexographic printingforms using such flexographic printing elements and also the use of theflexographic printing forms obtained for flexographic printing with UVinks.

The following details regarding the invention may be provided:

in the following, the term “flexographic printing form” or “flexographicprinting plate” is used for a previously crosslinked, ready-to-printprinting form. The term “flexographic printing element” is used in theusual way for the photopolymerizable starting material which is used forproducing flexographic printing forms or flexographic printing plates.

The photopolymerizable flexographic printing elements of the inventioncan be either plate-shaped flexographic printing elements orcylindrical, preferably continuous seamless flexographic printingelements.

The photopolymerizable flexographic printing elements of the inventioncomprise at least one dimensionally stable support and at least onephotopolymerizable, relief-forming layer.

Possible dimensionally stable supports are the supports known fromflexographic printing plate technology, for example films, plates orcylindrical tubes. The materials of the support can be, for example,metals such as steel or aluminum or plastics such as polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate orpolycarbonate. The supports can optionally have been treated withcustomary bonding layers. PET films having a thickness of from 100 to200 μm are particularly suitable.

The photopolymerizable material present in the photopolymerizable,relief-forming layer comprises at least one thermoplastic-elastomericblock copolymer, at least one ethylenically unsaturated monomer, atleast one photoinitiator, at least one plasticizer and optionallyfurther components.

The thermoplastic-elastomeric block copolymers comprise at least oneblock consisting essentially of alkenylaromatics and at least one blockconsisting essentially of 1,3-dienes. The alkenylaromatics can be, forexample, styrene, α-methylstyrene or vinyltoluene. Preference is givento styrene. The 1,3-dienes are preferably butadiene and/or isoprene.These block copolymers can be either linear, branched or radical blockcopolymers. In general, the block copolymers are triblock copolymers ofthe A-B-A type, but they can also be diblock polymers of the A-B type orcopolymers having a plurality of alternating elastomeric andthermoplastic blocks, e.g. A-B-A-B-A. It is also possible to usemixtures of two or more different block copolymers. Commercial triblockcopolymers frequently contain certain proportions of diblock copolymers.The diene units can be 1,2-linked or 1,4-linked. Furthermore, it is alsopossible to use thermoplastic-elastomeric block copolymers having endblocks of styrene and a random styrene-butadiene middle block. Ofcourse, it is also possible to use mixtures of a plurality ofthermoplastic-elastomeric binders, provided that the properties of therelief-forming layer are not adversely affected thereby.

In a preferred embodiment of the invention, thethermoplastic-elastomeric binders are at least one styrene-isopreneblock copolymer, in particular a styrene-isoprene-styrene blockcopolymer, with the polymers also being able to contain proportions ofdiblock styrene-isoprene copolymer. Preferred binders of thestyrene-isoprene type generally contain from 10 to 30% by weight,preferably from 12 to 28% by weight and particularly preferably from 14to 25% by weight, of styrene. These block copolymers usually have anaverage molecular weight M_(w) (weight average) of from 100 000 to 300000 g/mol. Of course, it is also possible to use mixtures of variousstyrene-isoprene block copolymers. In a second embodiment of theinvention, preference is given to using radial isoprene-styrene blockcopolymers.

The isoprene units in the polyisoprene blocks can be 1,4-linked, i.e.the remaining double bond is arranged in the chain, or 3,4-linked, i.e.the remaining double bond is arranged laterally. It is possible to useblock copolymers which have essentially 1,4-linkages and binders whichhave certain proportions of 3,4-linkages. The lateral vinyl groups inbinders having 3,4-linked isoprene units can likewise react during thecourse of crosslinking of the photopolymerizable layer and accordinglygive a plate having high crosslinking. For example, it is possible touse styrene-isoprene block copolymers which have a vinyl group contentof from 20 to 70%.

In a preferred embodiment of the invention, a radial styrene-isoprenecopolymer having a vinyl group content of less than 10% can be used. Ina second preferred embodiment of the invention, a mixture of twodifferent styrene-isoprene block copolymers is used. One of thesepreferably has a vinyl group content of at least 20%, in particular from20 to 70%, preferably from 25 to 45%. The other can have a lower vinylgroup content, for example a vinyl group content of less than 10%.Preference is also given to using a mixture of two styrene-isoprenecopolymers of which one has a high diblock content of more than 40% byweight and the second has a lower diblock content of 10-30% by weight.

Apart from the thermoplastic-elastomeric block copolymers mentioned, inparticular the styrene-isoprene block copolymers, the photopolymerizablelayer can also comprise further elastomeric binders different from theblock copolymers. Such additional binders, also referred to as secondarybinders, enable the properties of the photopolymerizable layer to bemodified. An example of a secondary binder isvinyltoluene-α-methylstyrene copolymers. The amount of such secondarybinders should normally not exceed 25% by weight, based on the totalamount of all binders used. The amount of such secondary binderspreferably does not exceed 15% by weight and particularly preferablydoes not exceed 10% by weight.

The total amount of binders is usually from 40 to 90% by weight based onthe sum of all constituents of the relief-forming layer, preferably from50 to 90% by weight and particularly preferably from 60 to 85% byweight.

The photopolymerizable relief-forming layer further comprises at leastone ethylenically unsaturated monomer. The monomers used should becompatible with the binders and have at least one polymerizable,ethylenically unsaturated group. As monomers, it is possible to use, inparticular, esters or amides of acrylic acid or methacrylic acid withmonofunctional or polyfunctional alcohols, amines, amino alcohols orhydroxy ethers and esters, esters of fumaric or maleic acid and allylcompounds. Preference is given to esters of acrylic acid or methacrylicacid. 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate,1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate ortrimethylolpropane tri(meth)acrylate are preferred. It is of coursepossible to use mixtures of various monomers.

The relief-forming layer preferably comprises at least one ethylenicallyunsaturated monomer having two ethylenically unsaturated groups, inparticular 1,6-hexanediol diacrylate and/or 1,6-hexanedioldimethacrylate.

The total amount of all monomers in the relief-forming layer together isfrom 1 to 20% by weight, preferably from 5 to 20% by weight,particularly preferably from 8 to 20% by weight, very particularlypreferably from 8 to 18% by weight and for example from 12 to 18% byweight, in each case based on the sum of all constituents of therelief-forming layer. The amount of monomers having two ethylenicallyunsaturated groups is preferably from 5 to 20% by weight, based on thesum of all constituents of the relief-forming layer, preferably from 8to 18% by weight.

The relief-forming, photopolymerizable layer further comprises at leastone photoinitiator or a photoinitiator system. Examples of suitableinitiators are benzoin or benzoin derivatives such as methylbenzoin orbenzoin ethers, benzil derivatives such as benzil ketals,acylarylphosphine oxides, acylarylphosphinic esters, polycyclic quinonesor benzophenones. The amount of photoinitiator in the relief-forminglayer is generally from 0.1 to 5% by weight, preferably from 1 to 4% byweight and particularly preferably from 1.5 to 3% by weight, based onthe amount of all constituents of the relief-forming layer.

According to the invention, the flexographic printing plate furthercomprises at least one cyclohexanepolycarboxylic ester of the generalformula R¹—(COOR²)_(n). Here, n is 2, 3 or 4, preferably 2 or 3 andparticularly preferably 2. R¹ is an n-linked cyclohexyl radical. Theradicals R² are each, independently of one another, a linear, branchedor cyclic, aliphatic hydrocarbon having from 3 to 20 carbon atoms,preferably from 4 to 18 and particularly preferably from 6 to 12 carbonatoms. Of course, it is also possible to use mixtures of variouscyclohexanepolycarboxylic esters as plasticizer.Cyclohexanepolycarboxylic esters, their preparation and use asplasticizers are known in principle. Examples of suchcyclohexanepolycarboxylic esters are given in WO 04/081127.

Preference is given to cyclohexanedicarboxylic esters of the formulaR¹—(COOR²)₂, where R¹ is in this case a divalent cyclohexane radical andR² is as defined above; these esters can be cyclohexane-1,2-dicarboxylicesters, cyclohexane-1,3-dicarboxylic esters andcyclohexane-1,4-dicarboxylic esters. Particular preference is given tocyclohexane-1,2-dicarboxylic esters.

Examples of particularly suitable cyclohexanepolycarboxylic estersinclude diisopropyl cyclohexane-1,2-dicarboxylate, di-n-hexylcyclohexane-1,2-dicarboxylate, diisohexyl cyclohexane-1,2-dicarboxylate,di-n-heptyl cyclohexane-1,2-dicarboxylate, diisoheptylcylohexane-1,2-dicarboxylate, di-2-ethylhexylcyclohexane-1,2-dicarboxylate, di-n-nonyl cyclohexane-1,2-dicarboxylate,diisononyl cyclohexane-1,2-dicarboxylate or di-n-dodecylcyclohexane-1,2-dicarboxylate. Further examples are given in WO04/081127, page 7, line 6 to page 14, line 14.

Very particular preference is given to using diisononylcyclohexane-1,2-dicarboxylate in the flexographic printing element ofthe invention.

In general, the cyclohexane-1,2-dicarboxylic esters used according tothe invention have alkyl radicals having from 3 to 20, preferably from 4to 18, particularly preferably from 6 to 12, in particular 9, carbonatoms, with the alkyl radicals being able to be branched or linear.Suitable cyclohexane-1,2-dicarboxylic esters having alkyl radicalshaving from 3 to 20 carbon atoms are listed in US 2006/0178446,paragraphs [0032] to [0042]. The cyclohexane-1,2-dicarboxylic esters canbe mixed esters, i.e. contain alkyl radicals having different chainlengths. In general, the alkyl radicals are branched, with numerousdifferent isomeric forms of alkyl radicals of the same chain lengthbeing able to occur in the cyclohexane-1,2-dicarboxylic esters. Thealkyl radicals of the cyclohexane-1,2-dicarboxylic esters used accordingto the invention are thus frequently isomer mixtures.

Suitable esters are, for example, the cyclohexane-1,2-dicarboxylicesters disclosed in WO 99/32427:

di(isopentyl)cyclohexane-1,2-dicarboxylate, obtainable by hydrogenationof di(isopentyl)phthalate having the Chemical Abstracts Registry Number(hereinafter: CAS No.) 84777-06-0;d(isoheptyl)cyclohexane-1,2-dicarboxylate, obtainable by hydrogenationof di(isoheptyl)phthalate having the CAS No. 71888-89-6;di(isononyl)cyclohexane-1,2-dicarboxylate, obtainable by hydrogenationof a di(isononyl)phthalate having the CAS No. 68515-48-0;di(isononyl)cyclohexane-1,2-dicarboxylate, obtainable by hydrogenationof a di(isononyl)phthalate having the CAS No. 28553-12-0, based onn-butene;di(isononyl)cyclohexane-1,2-dicarboxylate, obtainable by hydrogenationof a di(isononyl)phthalate having the CAS No. 28553-12-0 based onisobutene;a 1,2-di-C₉-ester of cyclohexanedicarboxylic acid, obtainable byhydrogenation of a di(nonyl)phthalate having the CAS No. 68515-46-8;a di(isodecyl)cyclohexane-1,2-dicarboxylate obtainable by hydrogenationof a di(isodecyl)phthalate having the CAS No. 68515-49-1;a 1,2-di-C₇₋₁₁-ester of cyclohexanedicarboxylic acid, obtainable byhydrogenation of the corresponding phthalic ester having the CAS No.68515-42-4;a 1,2-di-C₇₋₁₁-ester of ccylohexanedicarboxylic acid, obtainable byhydrogenation of the di-C₇₋₁₁ phthalates having the following CAS No.:111 381-89-6, 111 381 90-9, 111 381 91-0, 68515-44-6, 68515-45-7 and3648-20-7;a 1,2-di-C₉₋₁₁-ester of cyclohexanedicarboxylic acid, obtainable byhydrogenation of a di-C₉₋₁₁ phthalate having the CAS No. 98515-43-5;a di(isodecyl) 1,2-cyclohexanedicarboxylate, obtainable by hydrogenationof a di(isodecyl)phthalate which comprises mainlydi(2-propylheptyl)phthalate;a 1,2-di-C₇₋₉-ester of cyclohexanedicarboxylic acid, obtainable byhydrogenation of the corresponding phthalic ester which has branched orlinear C₇₋₉-alkyl ester groups; phthalates which can be used, forexample, as starting materials have the following CAS No.:di-C₇₋₉-alkyl phthalate having the CAS No. 111 381-89-6;di-C₇-alkyl phthalate having the CAS No. 68515-44-6; anddi-C₉-alkyl phthalate having the CAS No. 68515-45-7.

According to the invention, it is also possible to use hydrogenationproducts of mixed phthalic esters with C₁₀- and C₁₃-alcohols, as aredescribed in DE-A 10032580.7.

Particular preference is given to the dinonyl esters ofcyclohexane-1,2-dicarboxylic acid, for example the abovementioneddiisononyl esters or ester mixtures, or diisononyl 1,2-cyclohexanatewhich is commercially available under the name Hexamoll® DINCH from BASFSE.

In addition to the cyclohexane polycarboxylic esters, the flexographicprinting elements of the invention can optionally also contain furtherplasticizers. Examples of further plasticizers encompass paraffinic,naphthenic or aromatic mineral oils, synthetic oligomers or resins suchas oligostyrene, high-boiling esters, oligomeric styrene-butadienecopolymers, oligomeric α-methylstyrene-p-methylstyrene copolymers,liquid oligobutadienes, in particular those having an average molecularweight in the range from 500 to 5000 g/mol, or liquid oligomericacrylonitrile-butadiene copolymers or oligomericethylene-propylene-diene copolymers.

The amount of all plasticizers in the flexographic printing element ofthe invention is from 1 to 40% by weight and preferably from 1 to 20% byweight, with the proviso that the amount of allcyclohexanepolycarboxylic esters in the flexographic printing element ofthe invention is from 1 to 40% by weight, preferably from 1 to 20% byweight, particularly preferably from 1 to 10% by weight and for examplefrom 2 to 8% by weight. The absolute amount of plasticizers also dependson the respective binder system. In the case of plates based on astyrene-isoprene binder system, plasticizer amounts of from 1 to 10% byweight have been found to be useful. In an advantageous embodiment ofthe invention, exclusively cyclohexanepolycarboxylic esters are used asplasticizers. In the case of the styrene-isoprene binder system, lessplasticizer is generally required since the rubber containing generallyonly about 15-16% of styrene is softer than a styrene-butadiene rubbercontaining, for example, from 28 to 30% of styrene. In the case ofplates based on a styrene-butadiene binder system, plasticizer amountsof from 20 to 40% by weight have been found to be useful. In anadvantageous variant of this embodiment, the plasticizer can comprisefrom 1 to 10% by weight of cyclohexanepolycarboxylic esters with thebalance being made up by polybutadiene oils.

The relief-forming layer can additionally contain typical additives andadditional components. Examples of such additional components andadditives include dyes, inhibitors for thermal polymerization, fillersor antioxidants. A person skilled in the art can make an appropriateselection depending on the desired properties of the layers. However,the amount of such additional components should generally not exceed 10%by weight, based on the amount of all components of the relief-forminglayer, preferably not exceed 5% by weight.

The photopolymerizable flexographic printing element can of course alsobe a multilayer flexographic printing element which has two or more,generally two, photopolymerizable, relief-forming layers, with at leastone of the layers comprising at least one cyclohexane polycarboxylicester. A two-layer structure enables the bottom layer to be optimized,in particular, in respect of the elastic properties and the top layer tobe optimized in respect of the printing properties, for example the inkacceptance.

The photopolymerizable flexographic printing element can optionally alsohave an antiadhesive layer on the relief-forming layer, as is known inprinciple. Such a layer is used to prevent a photographic negative laidon top during illumination from sticking fast on the photopolymerizablelayer. An antiadhesive layer can, for example, comprise polyamide.

The photopolymerizable flexographic printing element can optionally alsohave a removable covering film, for example a PET film, on theantiadhesive layer to protect the photopolymerizable flexographicprinting element from damage.

The photopolymerizable flexographic printing elements of the inventioncan be produced by methods known in principle to those skilled in theart, for example by melt extrusion, casting or lamination in asingle-stage or multistage production process. They are preferablyproduced by means of melt extrusion, in which the constituents of therelief-forming layer are firstly mixed with one another with heating inan extruder. To produce sheet-like flexographic printing elements, thephotopolymerizable composition can be discharged from the extruderthrough a slit die between two films and the layer composite can becalendered, with the type of films depending on the desired use. Theycan be films which have good adhesion to the photopolymerizable layer orcan be readily removable (temporary) films. To produce sheet-likeflexographic printing elements, it is usual to use a firmly adheringsupport film and a removable covering film. If further processing of thelayer to give cylindrical flexographic printing elements by the processdescribed in the present invention is envisaged, then two removablefilms are used. To produce photopolymerizable, cylindrical flexographicprinting elements, a seamless layer can also be applied directly to acylindrical support by means of ring extrusion. The thickness of thephotopolymerizable layer is generally from 0.4 to 7 mm, preferably from0.5 to 4 mm and particularly preferably from 0.7 to 2.5 mm.Photopolymerizable flexographic printing elements for UV narrow webprinting generally have relatively thin photopolymerizable layers, forexample layers having a thickness of from 0.7 to 2.0 mm, preferably from1.0 to 1.8 mm.

The production of cylindrical continuous seamless flexographic printingelements and the further processing thereof to give continuous seamlessprinting forms can be carried out by a process based on that describedin WO 2004/092841. However, it can of course also be carried out bymeans of other techniques.

The further processing of the photopolymerizable flexographic printingelements to produce finished flexographic printing forms can be carriedout using various techniques. The flexographic printing elements can,for example, be illuminated in accordance with the image in a mannerknown in principle and the unilluminated regions of the relief-forminglayer can subsequently be removed by means of a suitable developingprocess. The illumination in accordance with the image can in principlebe carried out by covering the photopolymerizable flexographic printingelements with a photographic mask and illuminating thephotopolymerizable flexographic printing elements through the mask. Ifthe flexographic printing element is protected by a covering film, thisis removed beforehand.

However, the generation of the image is preferably carried out by meansof digital masks. Such masks are also referred to as in-situ masks. Forthis purpose, a layer on which an image can be generated digitally isapplied to the photopolymerizable, relief-forming layer. The layer onwhich an image can be produced digitally is preferably an IR-ablativelayer, ink-jet layer or thermographically inscribable layer.

IR-ablative layers or masks are opaque to the wavelength of actiniclight and usually comprise a binder and at least one IR absorber such ascarbon black. Carbon black also ensures that the layer is opaque. A maskcan be inscribed into the IR-ablative layer by means of an IR laser,i.e. the layer is decomposed and removed at the places at which thelaser beam strikes it. Examples for generation of images on flexographicprinting elements using IR-ablative masks are disclosed, for example, inEP-A-654 150 or EP-A-1 069 475.

In the case of ink-jet layers, a layer which can be written on by meansof ink-jet inks and allows transmission of actinic light, for example agelatin layer, is applied. A mask is applied to this by means of ink-jetprinters using opaque ink. Examples are disclosed in EP-A 1 072 953.

Thermographic layers are layers which contain substances which becomeblack under the action of heat. Such layers comprise, for example, abinder and an organic silver salt and an image can be produced on themby means of a printer having a thermal head. Examples are disclosed inEP-A 1 070 989.

The layers on which images can be generated digitally can be produced bydissolving or dispersing all constituents of the respective layer in asuitable solvent and applying the solution to the photopolymerizablelayer of the cylindrical flexographic printing element, followed byevaporation of the solvent. The layer on which images can be generateddigitally can be applied, for example, by spraying or by means of thetechnique described in EP-A-1 158 365.

The layers on which images can be generated digitally can also firstlybe applied in a separate coating step to a PET film which is then usedin the course of the production of the flexographic printing element bymeans of melt extrusion to produce a sheet-like layer composite in whichit is used as covering film in the calendering process.

After application of the layer on which images can be generateddigitally, an image is generated on this layer by means of a suitabletechnique and the photopolymerizable layer is subsequently irradiatedwith actinic light through the mask formed in a manner known inprinciple. Suitable actinic, i.e. chemically “active”, light is, inparticular, UVA or UV/VIS radiation. Lamps for illuminating plate-likeflexographic printing elements and also round lamps for the uniformillumination of cylindrical flexographic printing elements arecommercially available.

The layer which has been illuminated in accordance with the image can bedeveloped in a conventional way by means of a solvent or a solventmixture. Here, the unilluminated regions, i.e. the regions covered bythe mask, of the relief layer are removed by dissolution in thedeveloper while the illuminated regions, i.e. crosslinked regions, areretained. The mask or the residues of the mask are likewise removed bythe developer if the components are soluble therein. If the mask is notsoluble in the developer, it is, if appropriate, removed by means of asecond solvent before developing.

Developing can also be effected thermally. In thermal developing, nosolvent is used. Instead, the relief-forming layer after illumination inaccordance with the image is brought into contact with an absorbentmaterial and heated. The absorbent material is, for example, a porousnonwoven, for example of nylon, polyester, cellulose or inorganicmaterials. It is heated to such a temperature that the unpolymerizedcomponents of the relief-forming layer liquefy and are soaked up by thenonwoven. The fully soaked nonwoven is subsequently removed. Details ofthermal developing are disclosed, for example, by U.S. Pat. No.3,264,103, U.S. Pat. No. 5,175,072, WO 96/14603 or WO 01/88615. The maskcan, if appropriate, be removed beforehand by means of a suitablesolvent or likewise thermally.

The production of flexographic printing forms from thephotopolymerizable flexographic printing elements can also be carriedout by means of direct laser gravure. In this process, thephotopolymerizable layer is firstly crosslinked completely throughoutits entire volume by means of actinic light, electron beams or γ-rayswithout application of a mask. A printing relief is subsequentlyengraved into the crosslinked layer by means of one or more lasers.

The crosslinking over the entire area can be effected using customarylamps for flexographic printing forms as described above. However, itcan also be carried out particularly advantageously, especially in thecase of cylindrical, continuous seamless flexographic printing forms, bya process based on that described in WO 01/39897. Here, illumination iscarried out in the presence of a protective gas which is heavier thanair, for example CO₂ or Ar. The photopolymerizable, cylindricalflexographic printing element is for this purpose lowered into a dippingtank which is filled with protective gas and whose walls are preferablylined with a reflective material, for example aluminum foil. Thephotopolymerizable, cylindrical flexographic printing element issubsequently illuminated with actinic light. The customary UV or UV-VISsources of actinic light can in principle be used for this purpose.Preference is given to light sources which emit light having awavelength of from 200 to 400 nm. For example, it is possible to useconventional UV-A tubes, UV-C tubes, UV lamps or combinations thereof.

In direct laser gravure, the relief layer absorbs laser radiation tosuch an extent that it is removed or at least detached in the places atwhich it is exposed to a laser beam of sufficient intensity. The layeris preferably vaporized or thermally or oxidatively decomposed withoutprior melting, so that its decomposition products are removed from thelayer in the form of hot gases, vapors, smoke or small particles.

Lasers suitable for engraving the relief-forming layers used accordingto the invention are, in particular, lasers which have a wavelength offrom 9000 nm to 12 000 nm. Particular mention may be made here of CO₂lasers. The binders used in the relief-forming layer absorb theradiation of such lasers to a sufficient extent for them to be engraved.

The flexographic printing form obtained can advantageously be cleaned ina further process step after laser gravure. In some cases, this can beeffected by simple blowing-off with compressed air or by brushing.However, preference is given to carrying out such cleaning using aliquid cleaner in order to be able to remove even polymer fragmentscompletely. Suitable cleaners are, for example, aqueous cleaners whichconsist essentially of water and optionally small amounts of alcoholsand which may contain auxiliaries such as surfactants, emulsifiers,dispersants or bases to aid the cleaning process. “Water-in-oil”emulsions as disclosed by EP-A 463 016 are also suitable. Preference isgiven to using cleaners which have at least one organic component whichis able to detach the decomposition products deposited on the relief ofthe flexographic printing element during laser gravure without therelief layer being significantly swelled during the cleaning process.Such cleaners are disclosed, for example, in WO 2005/113240.

The flexographic printing forms produced by the process of the inventionhave very good flexibility. The effect of the cyclohexanepolycarboxylicesters used according to the invention is particularly pronounced here,especially in the case of relatively highly crosslinked, relatively hardflexographic printing forms.

The relief layer of the ready-to-print flexographic printing formsgenerally has a Shore A hardness in accordance with DIN 53505 of from 30to 90 Shore A, preferably from 50 to 85 Shore A and particularlypreferably from 60 to 85 Shore A and very particularly preferably from75 to 85 Shore A, at a layer thickness of 1.14 mm. The measurements arecarried out using a hardness measuring instrument, corresponding to thedescription of DIN 53505.

The ready-to-print flexographic printing forms can preferably be usedfor flexographic printing using UV inks. However, they can of coursealso be used for printing with other inks, e.g. conventionalflexographic printing inks based on water or alcohol.

For printing using the flexographic printing forms according to theinvention, in particular using the flexographic printing forms based onstyrene-isoprene binders, preference is given to using printing inkshaving comparatively polar monomers. Suitable inks are, in particular,UV printing inks containing acrylic esters based on polyether polyols,for example ethoxylated tripropylene glycol diacrylate, ethoxylatedtrimethylolpropane triacrylate, dipropylene glycol diacrylate ortripropylene glycol triacrylate.

For flexographic printing with UV inks, it is possible to usconventional flexographic printing machines which are equipped forprinting with UV inks. At least one flexographic printing form ismounted on a printing cylinder; for polychrome printing, three, four oreven more plates are used depending on the printing technique.Afterwards, UV-curable printing ink is transferred to the flexographicprinting form in a customary way by means of an inking unit, theUV-curable printing ink is transferred from the flexographic printingform to the print substrate by rotation of the printing cylinder and theUV-curable printing ink on the substrate is then cured by means of UVradiation. In the case of polychrome printing, a number of platescorresponding to the number of inking units is used.

The flexographic printing forms can preferably be used for narrow webprinting. For this purpose, preference is given to using printingmachines whose printing cylinder generally does not exceed a length of60 cm and has a diameter of from 5 to 15 cm. It can, for example, have alength of from 30 to 60 cm, in particular cases even only about 10 cm.This technique enables labels, in particular self-adhesive labels suchas stickers or adhesive labels made of paper and films, to be printed.Furthermore, it is possible to print, for example, wrap arounds, sleevefilms, inmold labels (for example for ice cream packaging), laminates(for example for toothpaste tubes), products for special industrialapplications (for example vignettes) or packaging in general made offilms, paper or board.

The following examples illustrate the invention.

EXAMPLES 1. General Method for Producing the PhotopolymerizableFlexographic Printing Elements

The components for producing the photopolymerizable layer wereintensively mixed with one another at a temperature of about 125° C. ina customary way in a twin-screw extruder, discharged in a customary waythrough a slit die and calendered between a dimensionally stable supportand a covering film.

As support, a conventional PET film coated with an adhesive coating andhaving a thickness of 175 μm was used, and a PET film provided with aconventional antiadhesive layer and having a thickness of 125 μm wasused as covering film. The composite of support layer and photopolymer,relief-forming layer had a total thickness of 1.14 mm in each case.

2. General Method for Producing the Flexographic Printing Plates

The photopolymerizable flexographic printing elements obtained asdescribed in 1) were in each case subjected to preliminary illuminationfor 50 s (or 20 s) from the rear side, subsequently irradiated throughan applied mask from the front with UV-A radiation under reducedpressure for 24 minutes, washed out (brush setting 0 mm) at a washoutspeed of 140 mm/min at 30° C. in a commercial washout solvent forflexographic printing plates (Nylosolv® A), dried at 65° C. for 2 hoursand after-illuminated with UV-A radiation for 10 minutes and UV-Cradiation for 20 minutes to eliminate stickiness. A commercial lamp forflexographic printing plates, viz. Nyloflex® lamp F III, was used forillumination.

To produce the long strips for flexibility measurements, illuminationwas carried out without application of a negative in the indicatedmanner to give a strip crosslinked over its entire area.

3. Determination of the Measured Values

The Shore A hardness and the flexibility were determined on each of theflexographic printing plates obtained. The measurements were carried outonce after production of the flexographic printing plates and a secondtime after a delay time of from 9 to 13 days.

3.1 Determination of the Shore Hardness

The Shore A hardness was measured in accordance with DIN 53 505 on the1.14 mm thick flexographic printing plates by means of a typicalhardness measuring instrument by means of which the measurements werecarried out in accordance with DIN 53 505.

3.2 Determination of the Flexibility

The principle of the measurement is shown schematically in FIG. 1.

To determine the flexibility, strips having a length of 30 cm and awidth of 2 cm are in each case cut from full areas of the flexographicprinting plate. The flexographic printing plate including support filmis used for the measurement. The strips of the flexographic printingplate (1) are adhesively bonded in a length of 15 cm to a solid support(2) by means of a double-sided adhesive tape. The unsupported part ofthe strip of likewise 15 cm is left to hang free and bends to a greateror lesser extent under the action of gravity. The angle α between thesupport point of the plate (3) and the free-hanging end of the plate andthe horizontal, fixed printing plate surface on the solid support ismeasured. For this purpose, any angle measuring device, for example thatdescribed in U.S. Pat. No. 4,766,675, can be used. The greater the angleα, the greater the flexibility of the flexographic printing plate andthe more easily can it be bent around a printing cylinder.

3.3 Determination of the Swelling Data

To determine the percentage increase in weight due to swelling, aduplicate determination is carried out in each case. Here, twoflexographic printing plate pieces having a size of about 1 cm×2 cm arecut, weighed and subsequently wetted completely at room temperature withthe monomer or the printing ink whose swelling behavior is to beexamined. Swelling is subsequently allowed to occur for 1 hour at 40° C.or 24 hours at room temperature. The specimens are subsequently cleanedof residues of adhering monomer or printing ink using a textile clothwithout solvent and weighed again. The swelling as a percentage increasein mass can be calculated from the difference. Furthermore, the Shore Ahardness is determined before and after swelling. The mean of twospecimens is formed in each case.

Swelling experiments were carried out using hexanediol diacrylate,dipropylene glycol diacrylate and commercial UV printing inks.

4. Starting Materials Used

Quintac ® radial styrene-isoprene block copolymer, styrene 3621 Ccontent from 14 to 15% by weight, diblock content about 26%, vinyl groupcontent from 7 to 8%. Hybrar ® SIS block copolymer having a styrenecontent of 20% 5125 by weight, proportion of vinyl groups about 35%.Kraton ® SIS block copolymer having a styrene content of from D 1113 BT15 to 17% by weight, proportion of vinyl groups from 5 to 8%, diblockcontent about 55% by weight, M_(w) about 250 000 g/mol. Hexamoll ®Diisononyl cyclohexane-1,2-dicarboxylate DINCH Polyöl ® Polybutadieneoil having a molecular weight M_(n) of 130: about 3000 g/mol, aviscosity of 2700-3300 mPas at 20° C. and a proportion of 1,2-vinylgroups of about 1%. Lithene ® Polybutadiene oil (Synthomer) having amolecular N 4 5000 weight M_(n) of about 5000 g/mol, viscosity about4000 mPas at 25° C., proportion of 1,2-vinyl groups of from about 10 to20%. Nisso ® Polybutadiene oil (Nippon Soda) having a molecular PBB-1000 weight M_(n) of 900-1300 g/mol, a proportion of 1,2- vinyl groupsof >85% and a viscosity of 5000-15 000 mPas at 45° C. Winog ® White oil70 Plastomol ® Diisononyl adipate DNA Monomers 1,6-hexanediol diacrylateor 1,6-hexanediol di(meth)acrylate Additives Dyes, inhibitor for thermalpolymerization

In accordance with the general method, the photopolymerizableflexographic printing elements described below were produced and in eachcase processed further to give flexographic printing forms. Thecomposition of the layer and the measured values obtained are shown intables 1 to 4.

Trial 1 (Example 1 and Comparative Examples C1, C2 and C3)

In trial 1, a formulation having a very high degree of crosslinking wasused. The formulation contains 15% by weight of monomers, a first SISbinder and 35% by weight of an SIS binder having a high proportion ofvinyl groups, i.e. groups which can likewise be crosslinked. The Shore Ahardness of the flexographic printing plates is from 86 to 88 in eachcase and thus very high. The formulations and results are shown in table1.

In all experiments, it was observed that the flexibility after a delayin time of from 1 to 2 weeks after production of the plate continues toincrease a little. The plate C1, which contains absolutely noplasticizer, has a flexibility of only 11° (after 9 days). If 7%polybutadiene oils is used as plasticizer (C2 and C3), a flexibility offrom 17° to 19° is obtained, but when diisononylcyclohexane-1,2-dicarboxylate is used according to the invention(example 1) a significantly better flexibility of 27° is obtained.

Although example 1 displays a significantly greater flexibility comparedto the comparative experiments C2 and C3, the swelling values are at thesame level. The swelling values in the case of the plate withoutplasticizer are somewhat lower. The swelling experiments also show thatthe more strongly polar monomer dipropylene glycol diacrylate swells theflexographic printing plates to a significantly lesser degree than theless polar monomer hexanediol diacrylate.

Trial 2 (Examples 2 and 3, Comparative Examples C3 to C6)

In trial 2, a formulation having a somewhat lower (compared to trial 1)but still high degree of crosslinking was used. The formulation contains12.5% by weight of monomers, a first SIS binder and 36% by weight of anSIS binder having a high proportion of vinyl groups, i.e. groups whichcan likewise crosslink. 7.0% or 2.5% of diisononylcyclohexane-1,2-dicarboxylate (examples 1 and 2) and in each case 7% ofother plasticizers, namely 2 different polybutadiene oils, a white oiland diisononyl adipate, were used. The formulations and results areshown in table 2.

The flexibility when using 7% of diisononylcyclohexane-1,2-dicarboxylate is 25%, and when using white oil orpolybutadiene oils, values of from 19° to 21° are achieved. Although aflexibility of 33° is achieved using 7% of diisononyl adipate, the ShoreA hardness is 83 and thus the lowest of all experiments.

After swelling in the acrylates HDDA, DPGDA and in UV ink, the moreflexible plate as per example 2 displays no significantly higher weightincrease during the course of swelling compared to C3, C4 and C5. Heretoo, the swelling experiments show that the relatively highly polarmonomer dipropylene glycol diacrylate swells the flexographic printingplates to a significantly lesser degree than the less polar monomerhexanediol diacrylate.

Trial 3 (Example 4, Comparative Examples C7 to C9)

In trial 3, a formulation having a comparatively low degree ofcrosslinking was used. The amount of monomers was 12.5% by weight, butonly binders low in vinyl groups were used. The initiator content wasrelatively low at 1.3% by weight. The Shore A hardnesses of theflexographic printing plates were accordingly only from 73 to 75. Theformulations and results are shown in table 3. No large differences inthe flexibility were observed.

Trial 4 (Example 5, Comparative Examples C10 and C11)

In trial 4, a formulation having a comparatively low degree ofcrosslinking was used. Although the amount of monomers was 18.8% byweight, only binders low in vinyl groups were used. The initiatorcontent was relatively low at 1.5% by weight. The Shore A hardnesses ofthe flexographic printing plates were accordingly only from 76 to 77.The formulations and results are shown in table 4.

No large differences in the flexibility were observed. The percentageincrease in weight during the course of the swelling test is generallyhigher than in the other trials because of the low crosslinking.

Trial 5 (Example 6, Comparative Examples C12 and C13)

In trial 5, the amount of monomers was in each case 12.5% by weight. Acomparatively highly crosslinked formulation was obtained by increasingthe initiator content to 2.3% by weight. In example 6, diisononylcyclohexane-1,2-dicarboxylate was used as plasticizer, whilepolybutadiene oils were used in the other two experiments. The amount ofplasticizer was in each case calculated approximately so that theflexographic printing plates had a comparable flexibility of in eachcase from 32° to 35°. The formulations and results are shown in table 5.

When using diisononyl cyclohexane-1,2-dicarboxylate, only 1.7% werenecessary, while 6% of the polybutadiene oils had to be used in eachcase. However, this reduces the Shore A hardness significantly to 77 or78, while the Shore A hardness was 81 when using diisononylcyclohexane-1,2-dicarboxylate.

The examples and comparative examples show that thecyclohexanepolycarboxylic esters used according to the invention lead toflexographic printing plates having a good flexibility. This appliesparticularly to highly crosslinked plates having a high hardness (ShoreA hardness >80) and particularly when using crosslinkable binders.

TABLE 1 Formulation for the photopolymerizable layer and results offmeasurement Component C1 C2 C3 Example 1 Binders SIS block copolymer(Quintac ® 3621 C) 40.9% 33.9% 33.9% 33.9% SIS block copolymer, highvinyl content (Hybrar ® 5125) 35.0% 35.0% 35.0% 35.0% Vinyltoluene -methylstyrene copolymer (Piccotex ® 100) 5.0% 5.0% 5.0% 5.0% MonomersHexanediol diacrylate 11.0% 11.0% 11.0% 11.0% Hexanediol dimethacrylate4.0% 4.0% 4.0% 4.0% Initiator Benzil dimethyl ketal 1.8% 1.8% 1.8% 1.8%Plasticizers Diisononyl cyclohexane-1,2-dicarboxylate — — — 7.0%Polybutadiene oil (Nisso ® PB B-1000) — 7.0% — — Polybutadiene oil(Lithene ® N 4 5000) — — 7.0% — Additives Dyes, inhibitor for thermalpolymerization 2.3% 2.3% 2.3% 2.3% Measured values Flexibility, angle α[°], immediately  7°  9° 10° 12° Flexibility, angle α [°], after 9 days11° 19° 17° 27° Shore A hardness after 1 day 88 87 87 87 Shore Ahardness after 9 days 88 87 87 86 results of the swelling experimentsComponent C1 C2 C3 Example 1 Swelling Swelling in hexanediol diacrylate:1 h, 40° C. 4.5% 4.9% 4.6% 5.0% experiments Swelling in hexanedioldiacrylate: 24 h, RT 12.8% 13.9% 13.4% 13.9% increase in mass Swellingin dipropylene glycol diacrylate: 1 h, 40° C. 1.7% 1.8% 1.6% 1.5%Swelling in dipropylene glycol diacrylate: 24 h, RT 3.2% 3.6% 3.4% 2.9%Commercial yellow UV printing ink: 24 h, RT 1.6% 2.0% 1.9% 1.6% SwellingHexanediol diacrylate: before swelling 87.5 87 86.5 87 experiments 1 h,40° C. 85 83 83.5 82.5 hardness (Shore A) 24 h, RT 78.5 77.5 77 76Dipropylene glycol diacrylate: before swelling 87 87 87 87 1 h, 40° C.85.5 85 85 84.5 24 h, RT 84.5 84 84 83 Commercial yellow UV beforeswelling 88.5 87 86.5 87 printing ink: 24 h, RT 68 64 65.5 63

TABLE 2 Formulation of the photopolymerizable layer and results ofmeasurements Example Example Component 2 3 C3 C4 C5 C6 Binders SIS blockcopolymer (Quintac ® 3621 C) 35.4% 37.9% 35.4% 35.4% 35.4% 35.4% SISblock copolymer, high vinyl content (Hybrar ® 36.0% 38.0% 36.0% 36.0%36.0% 36.0% 5125) Vinyltoluene - methylstyrene copolymer 5.0% 5.0% 5.0%5.0% 5.0% 5.0% (Piccotex ®100) Monomers Hexanediol diacrylate 9.2% 9.2%9.2% 9.2% 9.2% 9.2% Hexanediol dimethacrylate 3.3% 3.3% 3.3% 3.3% 3.3%3.3% Initiator Benzil dimethyl ketal 1.8% 1.8% 1.8% 1.8% 1.8% 1.8%Plasticizers Diisononyl cyclohexane-1,2-dicarboxylate 7.0% 2.5% — — — —Polybutadiene oil (Nisso ® PB B-1000) — — 7.0% — — — Polybutadiene oil(Polyöl ® 130) — — — 7.0% — — White oil — — — — 7.0% — Diisonoyl adipate— — — — — 7.0% Additives Dyes, inhibitor for thermal polymerization 2.3%2.3% 2.3% 2.3% 2.3% 2.3% Measured values Flexibility, angle α [°],immediately 13°  8° 10° 11° 10° 20° Flexibility, angle α [°], after 13days 25° 13° 21° 19° 21° 33° Shore A hardness after 3 days 85 87 85 8485 83 Shore A hardness after 13 days 85 88 86 85 85 83 results of theswelling experiments Example Example Component 2 3 C3 C4 C5 C6 SwellingSwelling in hexanediol diacrylate: 1 h, 40° 5.0% 4.8% 4.9% 4.8% 4.8%5.3% experiments Swelling in hexanediol diacrylate: 24 h, RT 14.0% 13.2%13.8% 13.5% 13.1% 14.3% increase in mass Swelling in dipropylene glycoldiacrylate: 1 h, 40° C. 1.6% 1.7% 1.5% 1.4% 1.5% 1.5% Swelling indipropylene glycol diacrylate: 24 h, RT 2.9% 3.0% 3.2% 3.2% 3.1% 2.3%Commercial yellow UV printing ink: 24 h, RT 1.6% 1.5% 1.9% 1.7% 1.9%1.4% Swelling Hexanediol diacrylate: before swelling 84 86.5 83 84.5 8482 experiments 1 h, 40° C. 79 82 78.5 80 78.5 77 hardness [Shore A] 24h, RT 72 75 72 74 73 69.5 Dipropylene glycol diacrylate: before swelling84.5 86.5 82.5 84 83.5 81.5 1 h, 40° C. 83 85 81 83.5 82 80 24 h, RT 8183.5 79 82 80.5 79 Commercial yellow before swelling 84 85 83.5 84.583.5 82 UV printing ink: 24 h, RT, specimen 83 84 82 84 82.5 81.5 clean,3 days storage repeated 24 h, RT 81 83 79 81.5 80 78.5

TABLE 3 Formulation of the photopolymerizable layer and results ofmeasurements Component Example 4 C7 C8 C9 Binders SIS block copolymer(Quintac ® 3621 C) 50.9% 50.9% 50.9% 50.9% SIS block copolymer (Kraton ®D 1113 BT) 20.0% 20.0% 20.0% 20.0% Vinyltoluene - methylstyrenecopolymer (Piccotex ® 100) 5.0% 5.0% 5.0% 5.0% Monomers Hexanedioldiacrylate 9.2% 9.2% 9.2% 9.2% Hexanediol dimethacrylate 3.3% 3.3% 3.3%3.3% Initiator Benzil dimethyl ketal 1.3% 1.3% 1.3% 1.3% PlasticizersDiisononyl cyclohexane-1,2-dicarboxylate 8.0% — — — Polybutadiene oil(Polyöl ® 130) — — 8.0% — Polybutadiene oil (Lithene ® N 4 5000) — 8.0%— — Diisononyl adipate — — — 8.0% Additives Dyes, inhibitor for thermalpolymerization 2.3% 2.3% 2.3% 2.3% Measured values Flexibility, angle α[°], immediately 43° 45° 43° 41° Flexibility, angle α [°], after 1 month44° 44° 45° 44° Shore A hardness after 1 month 75 74 73 75 results ofthe swelling experiments Component Example 4 C7 C8 C9 Swelling Swellingin hexanediol diacrylate: 1 h, 40° C. 9.0% 9.5% 9.4% 9.1% experimentsSwelling in hexanediol diacrylate: 24 h, RT 20.3% 22.1% 21.7% 20.6%increase in mass Swelling in dipropylene glycol diacrylate: 1 h, 40° C.2.7% 3.4% 3.7% 4.8% Swelling in dipropylene glycol diacrylate: 24 h, RT3.2% 3.6% 3.4% 2.9% Commercial yellow UV printing ink: 24 h, RT 3.2%4.3% 3.8% 3.0% Swelling Hexanediol diacrylate: before swelling 75 74 7374.5 experiments 1 h, 40° C. 68 68 67 68 hardness (Shore A) 24 h, RT 6262 61.5 62.5 Dipropylene glycol diacrylate: before swelling 75 74 7374.5 1 h, 40° C. 71 70 70 71 24 h, RT 69.5 69 68 69 Commercial yellow UVbefore swelling 74 71.5 72.5 74 printing ink: 24 h, RT, specimen 73 7271.5 73 cleaned, 3 days storage repeated 24 h, RT 68 69.5 68.5 69.5

TABLE 4 Formulation of the photopolymerizable layer and results ofmeasurements Component Example 5 C10 C11 Binders SIS block copolymer(Quintac ® 3621 C) 68.5% 68.5% 68.5% Vinyltoluene - methylstyrenecopolymer (Piccotex ® 100) 5.0% 5.0% 5.0% Monomers Hexanediol diacrylate13.8% 13.8% 13.8% Hexanediol dimethacrylate 5.0% 5.0% 5.0% InitiatorBenzil dimethyl ketal 1.5% 1.5% 1.5% Plasticizers Diisononylcyclohexane-1,2-dicarboxylate 5.0% — — Polybutadiene oil (Nisso ® PBB-1000) — 5.0% — Polybutadiene oil (Lithene ® N 4 5000) — — 5.0%Additives Dyes, inhibitor for thermal polymerization 1.2% 1.2% 1.2%Measured values Flexibility, angle α [°], immediately 41° 39° 41°Flexibility, angle α [°], after 12 days 48° 44° 48° Shore A hardnessafter 12 days 77 76 77 results of the swelling experiments ComponentExample 5 C10 C11 Swelling Swelling in hexanediol diacrylate: 1 h, 40°C. 5.6% 5.7% 6.0% experiments Swelling in hexanediol diacrylate: 24 h,RT 14.6% 14.9% 16.4% increase in mass Swelling in dipropylene glycoldiacrylate: 1 h, 40° C. 1.4% 4.6% 2.3% Swelling in dipropylene glycoldiacrylate: 24 h, RT 2.6% 2.9% 4.7% Commercial yellow UV printing ink:24 h, RT 3.1% 3.2% 3.0% Swelling Hexanediol diacrylate: before swelling76.5 76.5 78.5 experiments 1 h, 40° C. 71 71.5 73.5 hardness (Shore A)24 h, RT 66.5 66.5 67 Dipropylene glycol diacrylate: before swelling 7777 78.5 1 h, 40° C. 73.5 74 76.5 24 h, RT 73.5 74 75.5 Commercial yellowUV before swelling 76.5 76.5 78.5 printing ink: 24 h, RT 56.5 57.5 57.5

TABLE 5 Formulation of the photopolymerizable layer and results ofmeasurements Component Example 6 C12 C13 Binders SIS block copolymer(Quintac ® 3621 C) 76.2% 71.9% 71.9% Vinyltoluene - methylstyrenecopolymer (Piccotex ® 100) 5.0% 5.0% 5.0% Monomers Hexanediol diacrylate9.2% 9.2% 9.2% Hexanediol dimethacrylate 3.3% 3.3% 3.3% Initiator Benzildimethyl ketal 2.3% 2.3% 2.3% Plasticizers Diisononylcyclohexane-1,2-dicarboxylate 1.7% — — Polybutadiene oil (Nisso ® PBB-1000) — 6.0% — Polybutadiene oil (Polyöl ® 130) — — 6.0% AdditivesDyes, inhibitor for thermal polymerization 2.3% 2.3% 2.3% Measuredvalues Flexibility, angle α [°], after 20 months 32° 35° 32° Shore Ahardness after 20 months 81 78 77

1. A photopolymerizable flexographic printing element for producingflexographic printing forms for printing with UV inks, which comprisesat least A) a dimensionally stable support and B) a photopolymerizable,relief-forming layer comprising at least 1) from 40 to 90% by weight ofa thermoplastic-elastomeric block copolymer comprising at least oneblock which consists essentially of alkenylaromatics and at least oneblock which consists essentially of 1,3-dienes, 2) from 1 to 20% byweight of ethylenically unsaturated monomers, 3) from 0.1 to 5% byweight of photoinitiator and 4) from 1 to 40% by weight of at least oneplasticizer, where the amounts are in each case based on the totalamount of all components of the photopolymerizable layer, and whereinthe at least one plasticizer comprises from 1 to 40% by weight of atleast one cyclohexanepolycarboxylic ester of the general formulaR¹—(COOR²)_(n) where n is 2, 3 or 4, R¹ represents an n-valentcyclohexane radical, and R² represents, independently of one another, alinear, branched or cyclic, aliphatic hydrocarbon radical having from 3to 20 carbon atoms.
 2. The photopolymerizable flexographic printingelement as claimed in claim 1, wherein the plasticizer is acyclohexane-1,2-dicarboxylic ester R¹(COOR²)₂.
 3. The photopolymerizableflexographic printing element as claimed in claim 1, wherein R² is alinear, branched or cyclic, aliphatic hydrocarbon radical having from 6to 12 carbon atoms.
 4. The photopolymerizable flexographic printingelement as claimed in claim 1, wherein the plasticizer is diisononylcyclohexane-1,2-dicarboxylate.
 5. The photopolymerizable flexographicprinting element as claimed in claim 1, wherein thethermoplastic-elastomeric block copolymers comprise at least onestyrene-isoprene block copolymer having a styrene content of from 10 to30% by weight.
 6. The photopolymerizable flexographic printing elementas claimed in claim 5, wherein the thermoplastic-elastomeric blockcopolymers are radial styrene-isoprene block copolymers.
 7. Thephotopolymerizable flexographic printing element as claimed in claim 1,wherein the thermoplastic-elastomeric block copolymers are at least twodifferent styrene-isoprene block copolymers having a styrene content ofin each case from 10 to 25% by weight, with one of the two blockcopolymers comprising a polyisoprene block having a 1,2-vinyl groupcontent of at least 20%.
 8. The photopolymerizable flexographic printingelement as claimed in claim 1, wherein the amount of ethylenicallyunsaturated monomers is from 5 to 20% by weight.
 9. Thephotopolymerizable flexographic printing element as claimed in claim 1,wherein at least one monomer having two ethylenically unsaturated groupsis used.
 10. A process for producing a flexographic printing form forprinting with UV inks which comprises illuminating thephotopolymerizable relief-forming layer with actinic light in accordancewith the image and developing the illuminated, relief-forming layer orilluminating the entire area of the photopolymerizable, relief-forminglayer and engraving the printing relief into the illuminated relieflayer using a laser, wherein a photopolymerizable flexographic printingelement as claimed in claim 1 is used.